Magnetic resonance techniques are applied in nuclear magnetic resonance (NMR) experiments and magnetic resonance imaging (MRI). In magnetic resonance techniques, the response of nuclear spins in a magnetic field is observed following excitation by a radio frequency (RF) field. There are three basic types of the RF excitation: sequential, simultaneous and random. Currently available techniques for both NMR and MRI using these types of RF excitation have limitations.
Several different NMR techniques have been developed including: continuous wave (CW), pulsed, stochastic, and rapid scan correlation spectroscopy.
Using CW techniques, an RF sweep is run exciting different resonating frequencies as a function of time. Typically the sweep has a low RF amplitude requirement. The CW sweep is run very slowly to maintain a steady state and avoid saturation of the signal. Thus, a large RF pulse will saturate the system and cannot be used. If the continuous wave sweep is run at a faster speed, “wiggles” will appear in the acquired signal. The resulting signal (or acquired signal) is looked at as a function of frequency. Historically, the first high resolution spectra of liquid samples were acquired using the CW technique. NMR spectroscopy using the CW technique became an important tool in the field of chemistry until the early 1970's. However, due to the slow rate of acquisition, CW NMR is time consuming and thus impractical for in vivo or clinical applications.
Pulsed Fourier Transform (FT) spectroscopy achieves efficiency and sensitivity beyond that of the CW technique. Using pulsed Fourier spectroscopy, a short, intense pulse at a constant frequency is applied that excites a plurality of resonances simultaneously. Typically, the pulse is a high power monochromatic RF pulse and there is a limited range of excitation frequencies. The resulting signal (or acquired signal) is looked at as a function of time. In the pulsed FT technique, the excitation pulse is generally assumed to be short (a delta function), so that the FT of this input function can be considered approximately equal to unity, and thus, the system spectrum (H(ω)) can be obtained directly via FT of the impulse response (h(t)). FT spectroscopy can achieve high sensitivity because, during a given sampling time, a pulse may be repeated multiple times and the noise averaged. FT spectroscopy further has good flexibility insofar as the spin dynamic may be managed using a variety of different pulses.
Stochastic NMR uses a long series of small flip angle RF pulses whose phase or amplitude is modulated in a random manner. The spin system generally remains in the linear region. The technique typically involves several hundred pulses with a psuedo randomly determined phase. Phase is generally determined by Maximal Length Binary Sequence (MLBS). MLBS approximates white noise. Stochastic NMR has a broader range of excitation frequencies than does conventional FT pulsed NMR. Systematic noise inherent in images obtained with random noise excitation is eliminated by using pseudorandom noise together with Hadamard transformation for data evaluation. A back projection algorithm is used to reconstruct the images. A limitation in stochastic NMR is the need to create truly random excitation in order to avoid systematic noise artifacts. During excitation, a flip angle is used to define the angle of excitation for a field echo pulse sequence. The flip angle is the angle to which the net magnetization is rotated or tipped relative to the main magnetic field direction via the application of an RF excitation pulse at the Larmor frequency. The small flip angle used in stochastic NMR contributes to low sensitivity of the system. The acquired signal typically has a low amplitude. Further, hardware limitations frequently limit the utility of the system.
Rapid scan correlation spectroscopy combines the rapidity of FT NMR with the dynamic range of CW NMR. Rapid scan correlation spectroscopy uses a series of chirp pulses. The chirp pulses and the acquired pulses are correlated to produce a spectra. While rapid scan correlation spectroscopy is faster than CW NMR, its slow speed remains a limitation in its application.
In the evolution of high resolution NMR, pulsed Fourier NMR spectroscopy has continued to dominate the field, while the CW NMR technique essentially became obsolete and was used only for niche applications.
The above discussion relates specifically to high resolution NMR, typically performed in a laboratory setting on sample objects generally of test tube size. Another field of application for magnetic resonance techniques is clinical applications, in vivo applications, or MRI applications. MRI has technical requirements beyond those of NMR. Because the objects of interest (such as the brain or breast) is usually much larger than a test tube, the static and RF fields used in MRI are more inhomogeneous than those used in high resolution NMR. For applications on living systems, the need to avoid tissue heating places limits on the intensity and duty cycle of the RF irradiation that can be safely used, also known as the specific absorption rate (SAR) limitations. Similarly, when used in vivo, the subject must remain generally stationary during the MRI. Thus, slow techniques are difficult in clinical applications.
Several MRI techniques have been developed including CW MRI, pulsed FT MRI, including, Ultra-short Echo Time (UTE) MRI, and stochastic MRI. Additionally, rapid scan techniques have been considered for imaging using electron paramagnetic resonance (EPR).
Continuous wave MRI system using low RF power has been applied to study a variety of heterogeneous materials exhibiting very short relaxation values. As with CW NMR, CW MRI must be run at a very slow rate. This rate typically precludes application in clinical applications.
Pulsed FT MRI is useful in imaging of soft tissue but has limitations in the imaging of materials having fast relaxing spins (including semi-solid objects such as bone). These techniques use a short monochromatic RF pulse for excitation, followed immediately by signal acquisition, all performed in the presence of a field gradient. In MRI applications, the RF pulse can approach a delta function only by restricting the flip angle to <<90°. As the pulse length increases, the pulsed FT spectrum suffers from increasing phase and baseline distortions. Further, although these fast imaging sequences have minimal demands on gradient hardware performance, they are severely limited by the length of the pulse. To achieve broadband excitation with limited ω1 amplitude, a small flip angle is employed, which results in low S/N. Specifically, the width of the pulse leads to low sensitivity and low contrast in clinical applications. Artifacts such as increasing phase and baseline distortions can be avoided by replacing the free induction decay (FID) with an echo acquisition. Echo time is the time between the application of a pulse and the peak of the echo signal in spin-echo pulse sequences. However, the finite echo time used to form an echo leads to reduced sensitivity to fast relaxing spins and increased susceptibility to motion and flow artifacts. For imaging of objects having fast relaxing spins, the echo time must be short.
One of the pulsed FT MRI methods—Ultrashort Echo Time (UTE) was developed to image fast relaxing spins using FID acquisition. The UTE sequence employs a self refocused excitation pulse, followed immediately by a pulsed-on gradient and signal acquisition. UTE MRI has short “echo” times, which is good for imaging of fast relaxing spins. However, UTE pulse sequences use a growing gradient and simultaneous acquisition. The gradient slew rate is the rate of ascent or descent of a gradient from zero to its maximum amplitude, either positive or negative. The shorter the rise time, the faster the gradients and smaller echo time. As a result, UTE has better sensitivity to fast relaxing spins. Thus, it is generally desirable that the gradient have a fast slope. Using UTE MRI, there is a gradient limitation that limits the resolution of the images. The minimum duration needed to execute the excitation pulse and gradient switching are limited by the available RF amplitude and the gradient slew rate, respectively.
As discussed with respect to stochastic NMR, stochastic MRI has a small flip angle and thus has low sensitivity and low contrast in imaging.
While not currently implemented in MRI, rapid scan techniques have been used in electron paramagnetic resonance (EPR). These techniques have generally been considered unsuitable for MRI because of the following difficulties. Rapid scan utilizes a fast gradient echo sequence such as a series of chirp pulses. To avoid non-uniform weighting of the spectrum, the excitation frequency must be swept linearly (chirp pulse), which excludes the use of tailored frequency sweeps. Generally, an excitation profile in MRI applications must be rectangular. This is not readily achievable using short chip pulses and, thus, a rapid scan MRI using such chirp pulses would result in a relatively bad excitation profile. In rapid-scan FT techniques, the detector frequency is synchronous with the time-dependent transmitter frequency, and thus, correlation must be used to restore the undistorted absorption line by removing the “ringing” that persists following passage through resonance. In accordance with the Nyquist theorem, sampling in the frequency versus time domain places an upper limit on the sweep rate. Thus, rapid scan FT MRI, even if suitable for implementation, has an inherent speed limitation and its slowness limits use in clinical applications.
Using available techniques, it is difficult to image objects having a broad distribution of relaxation times, including very short spin-spin relaxation times, such as semi-solid objects including bone, macromolecules, and quadripolar nuclei. Accordingly, it would be desirable to have a technique to image materials having short spin-spin relaxation times that is sufficiently fast for in vivo application.